1. Field of the Invention
The present invention relates generally to an inner roof panel for a removable hardtop for a vehicle that is made with a sandwich-type composite structure having a cellular core and, in particular, to such an inner roof panel that is formed with varied thicknesses being provided in selected areas for special purposes.
2. Background Art
Panels of sandwich-type composite structures having a cellular core have favorable weight and strength characteristics. Such panels are in common use in shipbuilding, aircraft construction, and rail vehicle construction. In the field of aircraft construction, sandwich structure composite panels are made that are based on thermoset resins reinforced with glass fibers. Non-uniform mechanical stresses to which they are subjected sometimes makes it necessary to form local reinforcing plies at those places in the panels where the mechanical stresses are concentrated.
In order to impart the desired shapes to the panels, and to maintain the shapes, the glass fibers and the thermoset resin (in the form of pre-impregnates) are deposited layer-by-layer in a mold, and are then heated to high temperatures so as to cure (i.e. polymerize) the resin permanently.
The first step in making locally-reinforced panels is to define zones where stresses are concentrated in the resulting panels, such zones being defined either by real testing or by computer simulation. After the zones are defined reinforcing plies are added at those places so as to make it possible to withstand such stresses.
The reinforcing plies may have oriented mats of woven fabrics of glass fibers, carbon fibers, or natural fibers embedded in a thermoset resin. The orientation of the fibers is selected according to the location of the stresses. The reinforcing plies are cut out to a pattern using special machines, e.g. water-jet cutting machines. The reinforcing plies are disposed layer-by-layer in a mold, either manually or by means of a robot, with each ply having a preferred orientation.
The next step is a baking step. The baking step is the step having the longest duration of the method of making such pieces because the stack of layers must be heated throughout while in the mold.
After baking the layers of thermoset material and reinforcing fibers disposed in the mold are pressed in the mold by evacuating the mold. Such evacuation presses the materials against the die or the punch to form into the desired shape and remove surplus resin. The desired shape panel is obtained with the reinforcing fibers being throughly impregnated with the resin.
This “lamination” technique, and in particular the “laying up” operation, is characterized by a very low level of automation, and a large labor input. This lamination technique makes it possible to achieve high strength performance levels but requires rigorous monitoring of quality. This technique is very costly and cannot be used at the high production volumes required in many fields such as the automobile industry.
Plastics processing technology has enjoyed significant recent advances, such that traditional high-strength materials such as metals are being replaced with fiber composite materials. These materials are not only light, but also are flexible and durable.
Thermoplastic composites have potential for solving many of the problems associated with thermosets. For example, unlike thermosets, thermoplastic can be reshaped, welded, staked, or thermoformed. Furthermore, thermoplastic are generally tougher, more ductile, and have greater elongation than thermosets.